1. Field of the Invention
This invention relates to modulators, and more specifically to digital modulators for synthesizing phase-modulated carrier waves.
2. Description of the Prior Art
Apparatus that transmits digital data over communications channels by means of phase modulation techniques is well known in the art. For example, a transmitter for generating a phase-modulated carrier wave according to applied digital data is disclosed in U.S. Pat. No. 3,128,342 issued to P. A. Baker on Apr. 7, 1964. This transmitter comprises both digital and analog circuit elements. Specific analog elements disclosed are filters, envelope modulators, and summing amplifiers.
It is known to represent complex analog waves by a series of digitally encoded samples, for example, as in a pulse-code-modulation telephone system. It is also known to store sets of such digitally encoded samples in memory means for later retrieval and reconstruction of the original complex wave. For example, voice synthesis systems have been constructed in which spoken words are stored in memory as sequences of digitally encoded samples, and an analog voice signal is synthesized by sequentially applying the samples for selected words to a digital-to-analog converter.
In many applications, the recent development of low-cost digital integrated circuit elements has made possible the replacement of analog circuit elements with combinations of digital circuit elements. Digital circuit elements are typically more reliable, smaller, less costly, and use less power than the prior art analog elements. Thus, it is desirable to provide digital circuits to replace the prior art analog circuits, such as those described in the Baker patent, in modulators for generating phase-modulated carrier waves.
Prior art systems for digitally synthesizing analog waves have typically assembled sequences of digitally encoded samples representing selected stored wave segments, such as samples comprising words in the voice synthesis system referred to above. It might be considered that a system similar to that used for voice synthesis could be used in which carrier waves having different phases are simply substituted for the words. However, a satisfactory phase modulator should be capable of blending one carrier-wave segment into the next without the change in phase being too abrupt. Further, a satisfactory phase modulator must be capable of selecting the phase of each carrier-wave segment according to applied data so that the correct phase shift is generated in the carrier wave to encode each data element. The digital modulator contemplated by this invention includes digital means for blending successive carrier-wave segments, and digital means for selecting the phase of each carrier-wave segment according to the applied data elements.